Bone Marrow Sparing Volume Modulated Arc Therapy in Whole Lung Irradiation

Purpose: Whole Lung irradiation (WLI) is advocated in patients with the Ewing sarcoma and giant cell tumors presenting with pulmonary metastases. Traditionally, WLI was performed using anteroposterior-poseroanterior (AP-PA) fields.Acute hematological toxicity is one of the side effects of whole lung irradiation(WLI) leading to interruption of treatment. With the novel radiotherapy techniques like Volume Modulated Arc Therapy (VMAT) it is possible to minimise treatment associated haematological side effects by sparing bone marrow. We report a new technique of Bone Marrow sparing VMAT (MS-VMAT) for WLI.

Methodology: We describe a patient of Giant cell tumour , with pulmonary metastases, planned for WLI. We generated three plans using standard AP-PA, VMAT and MS-VMAT. Immobilisation mould for the patient was prepared in supine position with arms overhead using wing board. CT simulation was done with 3mm slices from mandible to pelvic brim using breath hold technique. Clinical target volume (CTV) of both lungs was contoured to cover the entire three-dimension bilateral lung(B/L) volume to which 1mm Planning target volume (PTV) was given (breath hold with Image guidance used). pulmonary nodules identified and contoured separately for boost. OAR’s contoured- heart, sternum, vertebral body, spinal cord, marrow and liver. Patient was planned for B/L WLI to total dose of 24Gy/16Fr/3weeks and boost to parenchymal lung nodules 28.8Gy/16Fr/3weeksh using VMAT and MS-VMAT. Plan was generated with Eclipse v15.5 treatment planning system, with AAA algorithm. The goal was >98% of PTV should receive >95% of prescribed dose and < 2% PTV should receive >110% of the prescribed dose. Dose volume constraints as per QUANTEC. Ribs were not given constraints due to dose spillage. To compare VMAT, MS-VMAT and AP-PA fields technique, we generated a standard AP-PA field plan using two equally weighted beams with the 6MV (megavolts) of photons (as used for VMAT and MS-VMAT), with >95% PTV receiving >95% of prescribed dose. For PTV, homogeneity Index (HI) was used for comparison of the three plans. HI calculated as (D2%-D98%)/D50%.

Results: The dose to PTV, OAR’s and HI is listed in Table. Dosimetric advantage was observed in MS-VMAT over standard VMAT technique in terms of minimizing dose to bone marrow. MS-VMAT showed more sparing of marrow thereby minimizing the Grade III hematological toxicity. Disease is stable with no marrow or lung toxicity, post one year of MS-VMAT.

Structure VMAT MS-VMAT. AP-PA
PTV
D2 % 26 23.9 23.6

D95 % 26.4 23.7 23.2

D98 % 27.28 22.96 22.2

Heart (Dmean) 15.4 18.4 23.6

Liver (Dmean) 10.1 12.2 11.2

Ribs (Dmean) 20.3 19.1 22.2

Sternum (Dmean) 20.7 11.9 24.6

Thoracic Vertebrae (Dmean) 21 12.5 17.5

Bone Marrow (Dmean) 21.4 10.9. 18.3

HI 0.1 0.13 .19

Conclusions: Effective bone marrow sparing is possible with MS-VMAT. Large scale randomized trials are needed to validate our results.

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